Hi Sabine @Arielle ,
Increase the F-stop Value (DOF also depends on the size of your model).
Also make sure that the Focus is at the right point; if you set the Eye as Focus Object, it will take the Origin of the eye as focal point. Place the Cursor on the eye (the default behavior is set to Surface Project, so the Cursor will be placed at the Surface of the eye) and then Add an Empty and use that Empty as the Focus Object.
